Enhancement of Selective Conversion in Spatially Patterned Reactors

Transcription

1 Reaction Kinetics and the Development of Catalytic Processes G.F. Froment and K.C. Waugh (Editors) Elsevier Science B.V. All rights reserved. 199 Enhancement of Selective Conversion in Spatially Patterned Reactors A. S. C6t~, W. N. Delgass, and D. Ramkrishna School of Chemical Engineering, Purdue University, West Lafayette, IN Abstract Dual-functionality is introduced into packed bed reactors via structured spatial patterns in order to circumvent the limitations inherent in by-product inhibited reaction systems. Simulation results demonstrate the ability of patterned reactors to vastly enhance the selective conversion of several systems, some involving undesirable side reactions which punish the system for operating too hot. Comparisons between well-mixed and layered patterns indicate that the temperature-tuning flexibility of the novel layered configuration offers significant performance improvements over the mixed bed configuration for systems involving temperature-sensitive reactions whose optimum temperatures vary widely. 1. INTRODUCTION Spatial patterns are structured reaction networks in which different "regions" of a reactor contain different catalysts so that as the reaction fluid navigates its way from one "region" to the next, it cycles between different reactions. The goal of these patterns is to introduce an auxiliary reaction system in order to enhance the selective conversion of the primary reaction system. More specifically, for systems involving a desired reaction that is restricted from proceeding in an ideal manner, patterns offer a strategy for circumventing the performance limitation by manipulating the reaction mixture. In general, patterns add design flexibility by allowing one to tailor a reactor that directs the reaction along a prescribed course. Figure 1. Pattern Configurations: (a) Well-mixed pattern (b) Layered pattern The "regions" of different catalysts may be of various lengths, and it is these characteristic length scales which define the patterned configurations of interest. The two patterns considered in this paper are (1) a uniformly-mixed pattern in which the two functionalities are distributed homogeneously and (2) a discrete pattern in which the two functionalities are organized as distinct layers (Figure 1).

2 200 Since one of the premises of this work is that this switching between reactions is advantageous, it is clear that the number of switches will affect the performance improvement. Based on this alone, one would anticipate the mixed configuration with its infinite number of switches between reactions to be superior to the relatively few switches of the layered configuration. However, the layered configuration possesses a distinct advantage over the well-mixed case because its broad zones may be maintained at different temperatures. Mixed catalyst beds are forced to compromise the performance of one catalyst or the other as the different catalysts are maintained at a common operating temperature. The layered pattern allows each reaction's operating temperature to be tuned to its own optimum so that conversion and selectivity can be maximized. This flexibility is critical in the case of highly temperature-sensitive reactions whose preferred temperatures vary widely. It is this feature which makes the layered pattern particularly interesting and allows us to take the concept of dual-functionality in a new, novel direction. A previous paper [1] investigated a system in which the restriction imposed on the primary reaction was due to chemical equilibrium. The exothermic, equilibrium-limited primary reaction: A+B<=~C+D (where D is the desired product) was enhanced by introducing an auxiliary drain-off reaction intended to eliminate by-product C. Studies on this simple two-reaction system indicated that both the layered and mixed bed schemes could vastly bolster conversion to the desired product. Furthermore, operating the mixed bed hot (thus elevating both reaction rates) was generally superior to switching back and forth between "hot" and "cold" zones in the layered configuration. The exception occurred when the equilibrium constant for the primary reaction fell dramatically with rising temperature so that the mixed bed was "punished" for being hot. This paper focuses on systems in which a side-product of the primary reaction inhibits the reaction rate. This situation is similar to the equilibrium-limited case in that, as the product accumulates, the primary reaction shuts off. Therefore, the secondary reaction is implemented to remove this constraint by eliminating the inhibitor. Unlike the equilibrium-limited case, there is no punishment for running the simple primary/auxiliary reaction system too hot. Therefore, undesirable side reactions are introduced into the reactor simulations in order to incorporate a realistic punishment for operating the reactor at excess temperature. Specifically, these undesirable side reactions compromise selectivity by either competing for reactants or degrading the desired product. Simulation results will demonstrate how the utilization of patterns allows one to bypass the selectivity constraints by tailoring a reaction network to minimize unwanted reactions. These productinhibited systems are also of interest because the nonlinear kinetics they involve may lead to reactor multiplicity that will significantly complicate the analysis. The superposition of multiplicity-induced patterns (in which different catalyst pellets operate on different steady state branches) on top of the spatial patterns described herein will be the subject of later work.

3 PLUG FLOW REACTOR ANALYSIS Several systems are considered in this investigation, and the analysis is initiated by studying a steady state pseudohomogeneous plug flow model which has been derived in detail in an earlier work [1]. The nondimensionalized material and energy balances are given by, respectively: dyi = sir i for i = 1, 2,...,n (1) dz do= ~6iRiB i _h(o_oc ) dz i=1 (2) where 6i is the volume fraction of the catalyst for the ith reaction, 0c is assumed constant, and n is the number of reactions in the system. 2.1 System 1 The simplest system examined involves only two reactions. The primary reaction creates desired product D and by-product C which inhibits further reaction: reaction 1: A + B ~ C + D (catalyst 1) The auxiliary reaction is added to remove the inhibitor: reaction 2: C + F ~ G (catalyst 2) allowing the primary reaction to proceed forward freely. For this simple system, the plug flow analysis is straightforward and will be explained in detail. For the sake of brevity, this detail will be withheld from the discussion of later systems. The reaction rates were chosen as: r 1 = klcacb r 2 =k2ccc F (3) (1 + KAC A + KBC B + KcC c)2 and then nondimensionalized according to the formalism described previously [1]. The volume fraction of each catalyst is constant with reactor position for the unpatterned (61 = 1 and ~ = 0) and mixed bed (61 = ~ = 0.5) cases while, for the layered configuration, zl = 1 and 6~ = 0 in the odd numbered zones, and the reverse is true in even numbered zones. Generally, the two reactions have different optimal temperatures so that the layered configuration switches between odd numbered zones fed material at 01 and even zones at 02. The mixed and unpatterned cases are evaluated for a feed at 01, 02, and the average of these temperatures (0ave) in order to determine which temperature is most appropriate for each. In this specific case (with two simple reactions and no further restraints imposed), the system always produces its highest D yield when run at high 0 because rates are increased and there is no penalty for being hot. Therefore, there is no reason to switch between

4 202 high and low 0 in the layered scheme, and the layered, mixed, and unpatterned configurations are all run at a common elevated temperature. In order to demonstrate the benefits of removing C without introducing the complications of temperature effects, this simple system is modeled under isothermal operation. Figure 2 plots the extent of reaction profiles for the different schemes. The well-mixed pattern converts 62.5% of the reactants to D, the layered 47.7%, and the unpatterned 23.4%. Clearly, both patterned reactors significantly enhance the yield over the unpatterned case. For this simple two-reaction network (absent of a penalty for operating too hot), the mixed bed, with its more frequent switching between reactions, is always superior to the layered pattern. Extent of Reaction Profiles ~ O.6 O.4 o.~ ~ layered extant layered extant 2 mixed extent mixed extent 2 ~ " ~ l Primary Reaction one... unpatterned extent 1 _ ~,,~=. ~ ~ = ~ ~ ~ ~ ~ ~ 9 me ~ :. 7 ; ' " '... '"i 1--"i Auxiliary Reaction one 0 0 O z Figure 2. Extent of Reaction Profiles for Isothermal System System 2 In real systems, it is frequently the case that as temperature goes up, performance suffers as other limitations such as catalyst stability, coking, or the introduction of unwanted side reactions restrict the maximum operating temperature. This effect is studied by modeling systems that involve one of two different types of undesirable reactions which turn on and dominate at high 0. The first of these reactions competes with the primary reaction by utilizing reactants nonproductively. The specific reactions for this system are: reaction 1: A + B ~ C + D (catalyst 1) reaction 2: A + B ~ E (catalyst 1) reaction 3: C + F ~ G (catalyst 2) Maintaining catalyst 1 at low temperature minimizes the rate of reaction 2 so that valuable reactants are not wasted. Therefore, the unpatterned reactor and primary reaction layers of the segmented pattern are fed cold fluid (at 01 = 0) and exposed to a coolant while the secondary reaction layers receive hot material (at 0~ = 0.2) and are hot-walled. Running the mixed bed cold compromises the rate of the auxiliary reaction, but running it hot activates reaction 2. The relative weight of these factors determines the appropriate mixed bed operating temperature. When reaction 2 is relatively inactive, the well-mixed pattern does best when run hot, and its performance surpasses that of the layered pattern. However, when reaction 2 is active, running the mixed bed hot may not be advantageous due to the

5 203 domination of reaction 2. Figure 3 shows that in such cases both patterns can offer performance improvements over the unpatterned reactor, but the layered configuration is vastly superior to the hot uniformly-distributed reactor. The arrows in Figure 3a mark the D yield at reactor's end for each configuration (layered = 64.9%, mixed = 34.5%, unpatterned = 20.0%). In this case, running the well-mixed reactor at a compromise temperature, 0ave, improves its product yield to 54.3%. However, the layered pattern still maintains a much higher selectivity as it wastes only 6.9% of the reactants while the mixed bed wastes 45.6%. (a) Extent of Reaction Profiles (b) Temperature Profiles 0 o_ o7 o.t~ 0 s 1 0 o. I OA o~ 0.9! _.. _ t, I--I layer= extent 2 ~lb layered mixecl I 9 I 9 I I layer~l oxl~mt 3 unpatterneo mixed extent I.... ix= extent 2 ~] mixed..,.,,... mlxeo extent 3 pattemeo,,m,.t "~ "I unpattemed unpattemeo extent 2 Figure 3. (a) Extent (b) Temperature Profiles for System 2 (0m= --02) Frequently, the 0-tuning flexibility of the layered pattern produces tremendously superior selective conversion regardless of the 0 at which the mixed bed is run. This is particularly the case when the unwanted reaction is very 0- sensitive (as denoted by a high activation energy) so that the penalty for operating hot is stiff. Figure 4 depicts such a scenario. Here, the mixed pattern produces its best yield at 0ave but is still significantly inferior to the layered scheme in that it generates less D (layered = 50.5%, mixed = 23.1%) while wasting over five times as much reactant (layered = 14.3%, mixed = 76.8%). (a) (b) Extent of Reaction Profiles Temperature Profiles 0 o o. I : o o.g 1 I layered extent I layered l--,,~- I layered extent 2 mrr~l (I I 9 I 9 layered extent 3 unpattemea mixed I mixecl exlent mixeo extent mlxea extent 3,~ unpatternexl... unpatternecj extent unpattern~l extent 2 Figure 4. (a) Extent (b) Temperature Profiles for 0-Sensitive System 2 (0m=- 0ave)

6 System 3 The second undesirable reaction implemented in the analysis is one which degrades the desired product D through further reaction with B. The complete system of equations is defined by reactions 1 and 3 of System 2 with reaction 2 replaced by: Reaction 2: D + B ~ E + C (catalyst 1) Once again, due to its high activation energy, reaction 2 generates a significant adverse effect only at high temperature. In this case, the punishment for operating hot is twofold as product is destroyed and additional inhibitor created. Clearly, then, it is critical to minimize this reaction so that running the mixed bed hot is frequently unacceptable. Figure 5 plots the results of a simulation involving a highly 0-sensitive degradation reaction that dominates at high 0. Here, the hot mixed bed actually performs worse than the unpatterned case because more than 90% of the D generated in the mixed bed is degraded at the elevated temperature. With its temperature-tuning flexibility, the layered pattern is able to offer a significant yield enhancement. While the mixed pattern's selective conversion can be improved for such 0-sensitive systems by operating it cooler, the layered pattern will generally offer superior performance regardless of the mixed bed's temperature. (a) (b) Yield of D Profiles ~ _~1 1 m m myered exlm111 i 9 9 iaywr ~ t 2 IIIIIIII layerer ~(tb1t 3, mixed extent 1," ",,, mixecl extent 2 nixed extent 3 unpstterned recent unpattemed extent o.5 o o~ o.s --- ymla: m~ea D y]elcl: unpa~ernecl Figure 5. (a) Extent (b) Product Yield Profiles for 0-Sensitive System 3 (0~= = 0ave) If R2 is only moderately 0-sensitive, both patterns can provide benefits over the unpatterned case. This is demonstrated by Figure 6, generated for a mixed bed operated at intermediate 0. For this system, both patterns nearly triple the product yield produced by the patternless reactor (layered = 36.6%, mixed = 33.9%, unpatterned = 12.8%). However, the layered pattern provides a superior selectivity by minimizing reaction 2, converting only a few percent of the reactants to unwanted E rather than the nearly 20% with the mixed configuration. It is not always the case that the layered pattern is superior. In fact, when reaction 2 is even less 0-sensitive or relatively inactive, the advantage of the mixed pattern's infinite number of reaction switches outweighs the 0-tuning advantage of the layered bed. For such situations, it is not uncommon for the mixed bed to double or even triple the yield enhancement provided by the layered pattern. It

7 205 follows, then, that the choice between patterns will largely depend on the 0- sensitivity of the reactions involved. (a) (b) Extent of Reaction Profiles Yield of D Profiles ( o_s 0 o. 1 o.z o.~s la,/erecl extant 2 / yield: mixed I II IIIII layered exl~t 3 D yleld: unpatterned mixed extent mixed exwnt 32 mixed extent... unpattemed e~terrt unpaltemed s~le~t 2 Figure 6. (a) Extent (b) Product Yield Profiles for System 3 (0m~ - 0ave) 3. AXIAL DISPERSION The plug flow analysis demonstrates the potential advantage of running product-inhibited systems with patterns. More importantly, it validates the value of the novel layered patterns proposed in this work. One concern with this analysis is that it assumes a discontinuous 0 profile in going from one layer to the next. Since this is an unrealistic idealization, it is necessary to assess the extent to which the layered pattern's advantages are compromised by the smoothing of the 0 profile by axial heat dispersion. This is done by conducting simulations based on a mixing cell model containing a specific mechanism for the backflow of heat between cells. The model equations and information about the solution strategy are presented in an earlier paper [1]. (a) (b) Extent of Reaction Profiles ji" 9 I= Temperature Profiles m Rxnl layere0 -.r ~ I m layered I 9 = 9 Rxn 2,ay~ ~ll lavcre~l ~ IElllll Rxn :3 layered u np~ittenled ---- ~.~ rob,= ~ mixed Rxn 2 mixed,-,,,,,, Rxn 3 mixed... Rxn 1 u.~,.~ 4 unpattemcd Rxn 2 unpattemer Figure 7. (a) Extent (b) Temperature Profiles for Mixing Cell Model with Figure 4 Parameters (0== - 0ave)

8 206 The mixing cell model is used to determine whether or not the layered pattern retains its supremacy (in cases where the plug flow model predicts it) despite axial dispersion. Applying Figure 4's parameters to the mixing cell model generates Figure 7. A comparison of Figures 4 and 7 reveals that the performance of the layered pattern is slightly compromised by the backflow of heat into the main reaction zones (D yield falls from 50.5% to 46.8% when axial dispersion is included in the analysis), but this effect cannot undo the large advantage promised by the plug flow analysis. When the plug flow analysis predicts only a slight advantage for the layered pattern over the mixed, the relatively small drop in the layered reactor's yield can be enough to tip the scales in favor of the mixed bed. The bottom line is that axial dispersion will have a slight adverse effect on the selective conversion of the layered pattern; but if the advantage predicted by the plug flow analysis is significant, it will be largely maintained. 4. CONCLUSION Spatially patterned reactors can provide conversion and selectivity enhancements over traditional unpatterned reactors for systems involving byproduct inhibition. The uniformly-distributed and layered patterns each have their own operational advantage. The well-mixed configuration benefits from more frequent switching between primary and auxiliary reactions. The layered pattern provides an added degree of flexibility by allowing the layers of different catalyst to be maintained at their own optimal temperatures. This temperature-tuning ability is particularly important when conducting highly temperature-sensitive reactions, and it is the degree of this temperature-sensitivity that is critical in determining which pattern strategy will be most advantageous for a particular system. 5. NOTATION Bi h Ri yi z 8i 0 Oc Dimensionless heat of reaction i Dimensionless heat transfer coefficient between fluid and reactor wall Dimensionless rate of reaction i Dimensionless extent of reaction i Dimensionless reactor coordinate Volume fraction of catalyst for reaction i Dimensionless temperature of reacting fluid Dimensionless temperature of heat transfer media 6. REFERENCES A. S. C6t~, W. N. Delgass, and D. Ramkrishna, Chem. Eng. Sci., submitted.